The present invention relates generally to resonator systems, and more particularly, to piezoelectrically-transduced polycrystalline diamond micromachined resonators and coupled resonator systems that may be used in RF oscillator, bandpass frequency filter and mass sensing applications.
Nanocrystalline diamond (NCD) is an emerging material with growing applications in MEMS driven by its superior mechanical properties such as high acoustic velocity, low acoustic loss, chemical stability, and very low wear rate. The use of polycrystalline diamond as an acoustic media for surface acoustic wave (SAW) devices has proven unparalleled in increasing the frequency of operation while relaxing the requirement on the lithographic resolution. See for example, T. Uemura, et al. “Low loss diamond SAW devices by small grain size poly-crystalline diamond,” IEEE Ultrasonics Symposium Proceedings, vol. 1, 2002, pp. 431-434. Capacitively transduced diamond disk resonators have also been showcased at GHz frequencies, increasing the resonance frequency by a factor of 2 compared to the same size resonators made of polysilicon. See for example, J. Wang, et al., “1.51-GHz nanocrystalline diamond micromechanical disk resonator with material-mismatched isolating support,” IEEE International Conference on Micro Electro Mechanical Systems (MEMS '04), 2004, pp. 641-644. However, very high motional impedance of these capacitive devices limits their system-level applications as they are required to interface with low impedance radio frequency (RF) electronics. A resonator with high impedance introduces excessive loss when used in a filter and requires multiple gain stages to sustain oscillation in an oscillator circuit, increasing the power consumption and design complexity.
Thin-film piezoelectric-on-silicon (TPoS) composite bulk acoustic resonators (CBAR) were initially introduced by the present inventors to tackle the high motional impedance problem in micromachined resonators while preserving high quality factors and frequencies. This is discussed by S. Humad, et al., in “High frequency micromechanical piezo-on-silicon block resonators,” Technical Digest of the IEEE International Electron Devices Meeting (IEDM '03), 2003, pp 39.3.1-39.3.4 and G. Piazza, et al., in “Voltage-tunable piezoelectrically-transduced single-crystal silicon resonators on SOI substrate,” IEEE International Conference on Micro Electro Mechanical Systems (MEMS '03), 2003, pp. 149-152, for example.
The large electro-mechanical coupling coefficient in a piezoelectrically-transduced resonator can potentially provide orders of magnitude lower motional impedance compared to a capacitive resonator at the same frequency. The underlying structural material in these devices improves the energy density, structural integrity, and resonance frequency.
Single crystal silicon resonators have previously been developed by the present inventors and others. Various references discuss resonator devices including U.S. Pat. No. 7,023,065, issued to Ayazi, et al. entitled “Capacitive Resonators and Methods of Fabrication,” U.S. Pat. No. 6,909,221 issued to Ayazi, et al. entitled “Piezoelectric on semiconductor-on-insulator microelectromechanical resonators,” a paper by Devoe entitled “Piezoelectric Thin Film Micromechanical Beam Resonators,” Sensors and Actuators, A 88; 2001 pp. 263-272, a paper by Piazza, et al. entitled “Voltage-Tunable Piezoelectrically-Transduced Single-Crystal Silicon Resonators on SOI Substrate,” in Proc. of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS '03), Koyoto, Japan, January 2003, and a paper by Humad, et al. entitled “High Frequency Micromechanical Piezo-On-Silicon Block Resonators,” Technical Digest of the IEEE International Electron Devices Meeting (IEDM '03); 2003, pp 39.3.1-39.3.4. However, there are no known references relating to piezoelectrically-transduced micromachined bulk acoustic resonators fabricated on a polycrystalline diamond film deposited on a carrier wafer.
Quartz crystal microbalance (QCM) mass sensors have found many applications in chemical and biological sensors. However, their relatively large size can limit the extent in which QCM sensors are used in microsystems to detect small traces of chemical or biochemical agents. In applications for which an array of mass sensitive sensors is required to distinguish between various types of molecules, QCM sensors fail to offer a compact and cost effective solution.
In recent years, micromachined resonant mass sensors with a much smaller form-factor have attracted a lot of attention to fill in the gap for arrayed and/or implantable gravimetric bio/chemical sensors. Cantilever beams and thin film bulk acoustic resonators are amongst the more successful realizations of micromachined mass sensors. Higher frequency of operation can potentially improve the sensitivity of these devices compared to QCM sensors, if high quality factors are maintained. Sensitivity to environmental parameters (e.g., temperature) is also an issue that needs to be addressed in order to facilitate robust operation of micromachined mass sensors. Complex actuation and readout mechanism is another bottleneck hindering widespread use of these devices in microsystems.
Capacitively-transduced lateral bulk acoustic resonant sensors have been developed by the present inventors in an effort to address some of the issues associated with micromachined mass sensors. See, for example, Z. Hao, R. Abdolvand and F. Ayazi, “A High-Q Length-Extensional Bulk-Mode Mass Sensor with Annexed Sensing Platforms,” IEEE International Conference on Micro Electro Mechanical Systems (MEMS '06), pp. 598-601, 2006. These devices demonstrated relatively high Q values in air at ˜12 MHz while minimizing the change in the effective stiffness of the structure imposed by absorbed mass. However, to increase the sensitivity, the device dimension needed to be scaled down, resulting in a reduced capacitive transduction area. Therefore, motional impedance of the device will increase, which translates to high power consumption and higher phase noise when interfaced with an oscillator circuit. More importantly, small capacitive air gaps are prone to blockage and squeeze film damping when exposed to environment.
It would be desirable to have piezoelectrically-transduced polycrystalline diamond micromachined resonator structures that may be used in RF oscillator, bandpass frequency filter and mass sensing applications, and the like.